Add api publish mutex

[randombenj]

At the moment it is posible to have multiple publish updates running at the same time. This is a problem as both processes try to update the same files and some unexpected
behaviour might occur.

By using a global mutex, only one publish can be updated at any time.

Fixes #1125

[codecov]

Codecov Report

Merging #1130 (ba22273) into master (71fd730) will decrease coverage by 0.00%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master    #1130      +/-   ##
==========================================
- Coverage   66.31%   66.30%   -0.01%     
==========================================
  Files         141      141              
  Lines       15910    15913       +3     
==========================================
+ Hits        10550    10551       +1     
- Misses       4610     4612       +2     
  Partials      750      750              

Impacted Files	Coverage Δ
api/publish.go	0.00% <0.00%> (ø)
context/context.go	68.18% <100.00%> (+0.07%)	⬆️

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[randombenj]

After some internal discussion, we noticed that this is actually not an ideal solution.
Even though it might work, it will block all publish operations for all publish endpoints.
There should be only one lock per publish endpoint.